In the drilling of boreholes, the injection of mud, and drilling mud, respectively, into the drill string or the borehole via the pump system serves several purposes. The drilling mud is used to remove the extracted drill cuttings and spoils, and to cool and lubricate the drilling tool or its drive located at the bottom of the borehole. The pressure generated in the borehole must be equalized with the gases and liquids present in the surrounding rock.
Extremely powerful pump systems are necessary for producing the required volumetric flow, which is in the range of 3000 l/min, and for generating the required pressure, which may be as high as 500 bar.
It is standard practice to produce pressure and volumetric flow via the motion of one or more displacement elements (pistons) which provide a periodically changing work space which is sealed from the outside.
These reciprocating piston machines are characterized in that they carry out the rotating motion of the rotary drive unit via a crank mechanism in an axial motion, which moves the piston. The design of the piston as a plunger represents one variation.
The work space is formed by the cylinder sleeve and the piston which travels therein.
In its specialized use in mud pumps, the cylinder sleeve has distinctive features compared to internal combustion engines or other piston pumps, for example.
On the one hand, these cylinder sleeves are subjected to greater wear due to the pumping medium, i.e., the drilling mud, which is extremely corrosive and in particular is abrasive due to solids.
On the other hand, the pressure and volumetric flow in a mud pump may be adjusted not just by varying the rotational speed, but also by varying the piston that is used. This is achieved by inserting various pistons together with matching cylinder sleeves into the pumps.
Thus, the following requirements are imposed on the cylinder sleeve and in particular on its fastening:
Due to wear as well as adjustments and changes to the system at different pressures and volumes, the fastening must allow short setup times and a low expenditure of effort.
The cylinder sleeve must be pressed with a defined contact force against the receptacle in order to ensure the function of the seal at that location. Both components form the high-pressure partial region of the system. Leaks in this region are hazardous as well as costly.
The working environment as well as the tool that is used must be characterized in the field of the drilling industry as extremely rough and harsh. In addition, the drilling mud that is used is generally corrosive. Thus, robustness and resistance to corrosion represents another requirement of a fastening device.
According to the known prior art, cylinder sleeves are connected to the pressure-conducting part of the mud pump, referred to as the water portion, using three different systems.
The first system has a wing clamp having a conical flank, for which the contact force is produced by a cone. The contact force is produced by screws which tighten the clamp around the cylinder sleeve and the receptacle. The straight flank of the clamp lies against the cylinder sleeve, and the conical portion of the clamp lies against the receptacle on the corresponding counterpart.
The wing clamps have the major disadvantage that they are very unwieldy, and the torque may be applied to the cylinder sleeve only in a very imprecise manner using a torque wrench. In addition, this factor varies greatly as a result of the conical surface, which may become worn or soiled (oily), which has a direct influence on the contact force that is being transmitted. Installing the clamp requires a great expenditure of force and also entails a certain risk of injury. Monitoring of the contact force is not possible without retightening.
The second system has a hydraulically pretensioned bolt having a spring clamp. In this system the supporting bolts are elongated using a hydraulic pump, or the hydraulic force acts against a separate spring assembly, thus causing a deflection. The system is screwed on without torque. After the pressure is released, the required contact force present in the system is solely mechanical.
The hydraulic system offers the major advantage of a precise contact force on the cylinder sleeve, which is also referred to as a liner. However, it is disadvantageous that the cylinder sleeve and the clamping element must be lifted in two crane hoisting operations, and therefore separately. In addition, a hydraulic pump and hydraulic fluid must be present. The hydraulic components and in particular the filler are susceptible to damage, or are subject to failure and the need for frequent repairs. Due to the more complex design, the system is approximately twice as expensive as the wing clamp having a liner receptacle. Here as well, subsequent monitoring of the contact force is not possible, although there is a risk of the cylinder sleeve or liner detaching or loosening as the result of vibration from the drilling operation or the drilling mud injection process.
The third system clamps the cylinder sleeve by means of an external thread. The clamp has an element with an external thread which is placed over the cylinder sleeve and is supported on same. These elements are jointly screwed into an internal thread provided on the water portion, the required contact force being applied as a result of the pitch of the thread.
This system has a very simple mechanical design and is also easy to operate. A very uniform contact force is applied to the seal via the thread. However, the only possibility for applying torque, which, however, is very imprecise, is for the nut to be continuously struck with a hammer with the full strength of the installer. The only monitoring is leak inspection during operation. The thread is also susceptible to corrosion and damage, which together with the accompanying wear results in even greater variation of the contact force. In addition, the strength of the installer is a factor that is not easily calculated.
DE 3831909 A1 discloses a hydraulic cylinder having a cylindrical pipe and an end flange with a guide bore for a piston rod. The end flange is inserted into the cylindrical pipe using a plug-in socket. The cylindrical pipe is surrounded by a clamping ring in the region of the plug-in socket. In addition, mounting holes for accommodating clamping screws are provided, orthogonal to the longitudinal axis of the guide bore, which pass through the clamping ring, the cylindrical pipe, and the plug-in socket. An end flange/cylindrical pipe connection may thus be easily and quickly established in a modular system composed of prefabricated component parts, thus saving on manufacturing costs. This pipe/flange connection is used as a pneumatic or hydraulic motor, in particular as a drive for swivel arms in the field of robotics. In one particular design, it is described that the end flange and the plug-in socket are designed as a one-part component, and have a shared guide bore for the piston rod, sealing rings being supported in the guide bore and on the plug-in socket, resulting in proper sealing of the plug-in socket with the cylindrical pipe and the piston rod in the piston rod guide. During installation, the clamping ring is prevented from twisting by means of a centering pin on the cylindrical pipe for introduction of the mounting holes, by means of which the centering pin penetrates through the cylindrical pipe until reaching the plug-in socket. DE 3831909 A1 also discloses that a closed end flange without a guide bore for the piston rod is clamped with its plug-in socket in the other end of the cylindrical pipe in the same manner as the end flange having a guide bore for the piston rod, forming a cylinder cover and base.
The disadvantage of DE 3831909 A1 is the guiding of the piston rod in the cylinder, resulting in a statically indeterminate bearing of the piston rod, which causes increased wear on multiple wearing parts. This guiding also requires time-consuming replacement of the piston. Rapid installation and deinstallation of the piston and the cylinder sleeve is therefore not possible. In addition, the supplying of pressure media is complicated, and cannot be used in a pump having an integrated pressure media supply system. Another disadvantage is the action on the described pipe/flange connection, which is directed orthogonally with respect to the longitudinal axis of the plug-in socket.
U.S. Pat. No. 4,981,401 discloses an adjustable clamping screw having an adjustable stress level. The publication describes a screw which is introduced into an axial cavity, by means of which the stress within the component may be measured. The screw also includes a display which indicates, as soon as the screw is in use, that a preset value has been reached.
The adjustable stress-measuring bolt disclosed in U.S. Pat. No. 4,981,401 has a complicated mechanism which allows incorrect operation, and which due to the prevailing harsh conditions is not suitable for applications for conveying drilling fluid in the drilling of boreholes.
WO 2008/074428 A1 describes a fluid processing machine for compressing or conveying fluids, preferably for compressing gases to high pressures, having a linear motor, at least one cylinder, a solid piston which is axially movable in the cylinder or an axially movable liquid piston, and at least one compression space which is formed between the cylinder and the solid piston or the liquid piston. The linear motor transfers a translational driving force to the solid piston or to the liquid piston. In the described fluid processing machine, the leakage-free and lubricant-free compression and conveying of fluids at high pressures is made possible by a simple design, the solid piston being translationally driven by the traveling magnetic field of the linear motor which is generated by coils.
The cited document does not disclose a particular fastening of the cylinder of the fluid processing machine which is suitable for ensuring short setup times and a low expenditure of effort, especially in a harsh environment.